A coating layer formed around a golf ball main body is required to satisfy the following requirements: 1) the coating layer has high impact resistance (i.e., peeling resistance) so that it adapts to the deformation of the golf ball main body (i.e., the part of the golf ball other than the coating layer) when the golf ball is hit; and 2) the coating layer has high resistance to scuffing so that it is hard to get scuffed or cracked even when it is hit with a metal head like a short iron and receives high impact and friction. In general, when a golf ball is hit, it deforms 30 percent or more in its diameter. Therefore, if the coating layer has poor adhesion to the golf ball main body, or has excessively high hardness with poor elasticity, a problem arises that the coating layer does not adapt to the deformation of the golf ball main body, and peels off. On the other hand, if the coating layer is too soft, a problem arises that the coating layer easily gets scuffed or cracked. In addition, the coating layer is required to satisfy the above-described requirements not only in a dry condition but also in a wet condition, because a golf ball frequently gets wet with water during a golf play. When a golf ball gets wet, foams are generated in the coating layer or in an interface between the coating layer and the golf ball main body. The foams cause the decrease in the hardness of the coating layer, or the deterioration of the adhesion between the coating layer and the golf ball main body.
In order to form a coating layer excellent in resistance to impact and scuffing, conventionally used is a two-part urethane-based coating material produced by mixing polyol as a chief material and polyisocyanate as a curing agent. Thus-obtained coating material is applied to an outside surface of a golf ball main body, and then is baked to promote a curing reaction.
In general, a golf ball main body includes a core and a cover. A cover is generally produced using an ionomer resin. When an ionomer resin is heated to high temperature, the ionic bond of the ionomer resin is weakened. Therefore, if the coating material applied to the golf ball main body is dried at high temperature, there is a likelihood that the elasticity and the hardness of the cover made of the ionomer resin are deteriorated. As one of the methods to solve this problem, it is considered to dry the coating material at low temperature. However, at low temperature, the curing reaction proceeds slowly, and it takes long time to dry the coating material. This results in low productivity. For example, Japanese Unexamined Patent Publication No. 8-182775 discloses an urethane-based coating material produced by mixing polyol and polyisocyanate. In this prior art, in an attempt to improve adhesion between the coating layer and the cover of the golf ball, polyol and polyisocyanate are mixed with each other in such a manner that the amount of hydroxyl groups of polyol is excessive with respect to the amount of the isocyanate groups of polyisocyanate by molar ratio. However, the urethane-based coating material has a problem that it requires long drying time, specifically 24 hours or longer, at 40 to 50.degree. C. This results in low productivity. Under such situations, developments are being made in an urethane-based coating material which is dryable in a short time.
Recently, there has been developed an urethane-based coating material which is dryable in a short time. In the urethane-based coating material, a part of polyol has been already reacted with polyisocyanate to form prepolymer (hereinafter, referred to as urethane polyol).
For example, Japanese Unexamined Patent Publication No. 9-59566 discloses a coating material for golf ball including urethane polyol. The urethane polyol has weight-average molecular weight of 10000 to 50000, and has urethane bonds as main chains and hydroxyl groups as terminal groups. In the coating material, a part of hydroxyl groups of the polyol has been already reacted with isocyanate groups to form urethane. Thus-obtained coating material is dryable in a short time with high productivity. However, the presence of the urethane polyol results in poor adhesion of the coating material to the outside surface of the golf ball main body. This is because this coating material has the smaller amount of free hydroxyl groups and free isocyanate groups than the ordinary urethane-based coating material, and therefore, does not sufficiently react with the outside surface of the golf ball main body. The problem of poor adhesion is remarkable when the coating material is applied to a golf ball main body in a one-piece structure having a rubber core but no ionomer cover, because this coating material is intended to be applied to a cover. In addition, there is another problem that, when the coating material is applied to a one-piece golf ball and is dried at high temperature, the physical properties of the rubber core is deteriorated. In order to solve these problems, there are demands for developing a coating material which is dryable in a short time at relatively low temperature.